Proc. Ind. Acad. Sci., 1955, 41A 130 and 137 disclose circular polarisers and quarter waveplates made of combinations of birefringent plates to provide improved achromaticity. The individual retarders are combined with different azimuthal orientations of their optic axes to achieve the improvement in achromatic performance.
Seki et al, Mol. Cryst. Liq. Cryst., 1995, 263, 499 and Seki et al, Eurodisplay, 1996, 464 disclose a liquid crystal device (LCD) of the reflective electrically controlled birefringence (ECB) type comprising a nematic liquid crystal and a quarter waveplate. The optic axis of the quarter waveplate is crossed with that of the nematic liquid crystal and is at 45.degree. to a polariser disposed on one side of the nematic liquid crystal. The untwisted liquid crystal and quarter waveplate are disposed between the polariser and a mirror and a normally white state is achieved with the liquid crystal providing a retardation of .lambda./4. A black state is achieved by controlling the liquid crystal so as to provide zero retardation. This black state is effectively provided by the quarter waveplate and is achromatic only to the degree that the quarter waveplate is achromatic. A further retarder of negative birefringence with its optic axis perpendicular to its plane may be included to improve viewing angle performance.
Uchida et al, Asia Display, 1995, 599 discloses a reflective display in which a polariser and a mirror are disposed on opposite sides of a Hybrid Aligned Nematic Liquid Crystal Layer and a Retarder. The retarder is biaxial having an optic axis out of the plane to improve viewing angle and an in-plane optic axis which co-operates with the retardation of the liquid crystal layer to provide black and white states. The retarder in-plane axis is at 45.degree. to the polarising axis of the polariser and is crossed with the optic axis of the liquid crystal layer. A normally black state is provided when the net retardation of the liquid crystal layer and the retarder is equal to .lambda./4. A white state is provided when the retardation of the liquid crystal is such that it cancels the retardation of the retarder. Such a display depends for its achromaticity on optimising the dispersion of the optical elements.
Ishinabe et al, Eurodisplay, 1996, 199 discloses a full colour reflective LCD in which a HAN liquid crystal layer and a biaxial retarder are disposed between a linear polariser and a mirror. The optic axis of the HAN layer is crossed with the optic axis of the retarder and is at 45.degree. to the polarising axis of the polariser. A normally black state is achieved by making the difference between retardations of the liquid crystal layer and the retarder equal to .lambda./4. Achomaticity is improved by adjusting the dispersion of materials of the liquid crystal layer and the retarder so that the birefringences partially compensate each other.
Kuo et al, Asia Display, 1995, 135 also discloses an LCD in which a HAN liquid crystal layer and a biaxial retarder are disposed between a linear polariser and a mirror. A dark state is provided when the total retardation is equal to an odd number of quarter wavelengths. The retarder has an in-plane optic axis which is crossed with the liquid crystal axis and is at 45.degree. to the polariser axis. The display may be operated in the normally black mode, as described hereinbefore, or in the normally white mode, in which case a larger passive retarder is used. Achromaticity may be improved by optimising the dispersion of the elements or by adjusting the dispersion so that the birefringences compensate each other.
Wu et al, Applied Physics Letters, 1996, 68, 1455 discloses a reflective LCD in which a twisted nematic liquid crystal cell and a retarder are disposed between a polariser and a mirror. The twisted nematic liquid crystal cell has a twist angle of 90.degree., is relatively thin, and has its input director angled at 20.degree. to the axis of the polariser. The retarder provides a retardation of .lambda./4 and has an optic axis angled at 45.degree. to the polariser axis. The cell operates in the normally white mode where the retardations of the liquid crystal cell and the retarder cancel each other in the white state and the black state is obtained by reducing the retardation of the liquid crystal cell to zero. Accordingly, the achromaticity depends on the achromaticity of the retarder.
Kuo et al, Eurodisplay, 1996, 387 discloses a similar twisted nematic display which is operated in the normally white mode and again achieves a black state whose achromaticity depends on the achromaticity of the retarder.
Fukuda et al disclose in three papers (IDRC, 1994, 201; SID Journal, 1995, 3, 83; Asia Display 1995, 881) a reflective supertwisted nematic (STN) LCD comprising a single polariser and a single retardation film. The twist of the liquid crystal is between 220 and 260.degree. and the device operates in the normally white mode. STN liquid crystal is used to allow high multiplex ratios i.e: small voltage differences between on and off voltages and hence a large value of d.DELTA.n of the liquid crystal, for instance greater than 0.6 micrometers. Achromaticity is improved by varying the dispersive properties of the liquid crystal and the retarder.
EP-A-0 389 240, on which the preamble of claim 1 is based, discloses a projection system that uses a polarising beam splitter. Light incident on the polarising beam splitter is directed towards a reflective liquid crystal device. A .lambda./4 retarder is disposed between the polarising beam splitter and the reflective liquid crystal device.
EP-A-0 389 240 also discloses a full colour projection system, in which white light is split into red, green and blue components. Three polarising beam splitters are provided, one each for receiving red, green and blue light. Light incident on each of the polarising beam splitters is directed to a respective reflective liquid crystal device via a .lambda./4 retarder.